Fix JSON-RPC correctness bugs and implement loadProblem

[unp1]

Fixes five correctness bugs in the KeY JSON-RPC server (keyext.api) and the hand-rolled Java client (keyext.api.client), and implements loadProblem:

• loadProblem previously returned a future resolving to null (NPE-ing callers). Now implemented: a ProblemDefinition (sorts/functions/predicates + antecedent/succedent sequent) is rendered to a .key document — the sequent encoded as (a1 & … & an) -> (s1 | … | sm) — and loaded through the existing .key path.
• clientApi NPE race — null until setClientApi ran, so startup callbacks could NPE. Now initialised to a no-op ClientApi, swapped for the real proxy on connect.
• KeyIdentifications — plain HashMaps under multi-threaded request dispatch could lose updates / throw ConcurrentModificationException; now ConcurrentHashMaps.
• RPCLayer queue — add() threw IllegalStateException (killing the reader thread) on a full queue; now put() for back-pressure.
• RPCLayer partial reads — a single read() could truncate a message on a socket; now loops until the full message or EOF.

Tests (run against KeY 3.0.0-SNAPSHOT): new RPCLayerTest cases (partial reads, back-pressure), KeyApiImplTest (loadProblem rendering + end-to-end load incl. abstract sorts), KeyIdentificationsTest (concurrent register/query via Mockito). Also wired JUnit into the client module and repaired its stale test sources so the suite compiles. All green.

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